Nearly 10 million U.S. children had developmental disabilities in 2008 -- a 17% increase in just over a decade, says epidemiologist Sheree Boule, PhD, of the CDC's National Center for Birth Defects and Developmental Disabilities.

"Seventeen percent may not sound like a large number, but that represents about 1.8 million more children," Boule tells WebMD.

The figures are based on a nationally representative sample of parent-reported diagnoses of developmental disabilities, which included autism, ADHD, cerebral palsy, seizures, stuttering and stammering, moderate to profound hearing loss, blindness, learning disorders, and other developmental delays.

Most of the increase occurred in autism and ADHD, but it is not clear if the incidence of these disorders is substantially rising or if the numbers reflect an increased emphasis on early diagnosis and treatment, greater awareness of the conditions by parents and teachers, and a broadening of the diagnostic criteria.

Boule says the fact that more women are having babies later in life and more babies are being born preterm may be contributing to the rise.

Growing Awareness of ADHD and Autism

The survey results, published in Pediatrics, revealed that between 1997 and 2008 the proportion of U.S. children with one or more developmental disabilities increased from about 13% to 15%.

Among the other findings:

Boys were almost twice as likely as girls to have a developmental disability, and more than twice as likely to have a diagnosis of ADHD (9.5% vs. 3.7%).

The proportion of children with a diagnosis of autism increased by almost 300% during the 12-year period, from 0.2% of children to 0.7%.

Hearing loss diagnoses declined during the period studied, with much of the decrease occurring between 2006 and 2008.

Children from low-income families and children with public health insurance had a higher prevalence of ADHD and other disabilities, with the exception of autism.

Pediatrician Nancy Murphy, MD, who chairs the American Academy of Pediatrics council on children with disabilities, says the increase can be largely explained by greater awareness of health providers, parents, and teachers about the signs and symptoms of ADHD, and the recognition that autism represents a spectrum of related disorders.

Murphy is an associate professor of medicine at the University of Utah School of Medicine.

"Many kids who might previously have slipped through the cracks are being diagnosed," she tells WebMD.